1. Field of the Invention
This invention relates generally to the measurement of the concentration of an analyte in a fluid mixture, and more specifically relates to the measurement of the concentration of oxygen in a fluid sample which may also contain halothane.
2. Description of the Related Art
As life support systems have become more sophisticated, it has become important to provide real time monitoring of critical patient body function parameters. Among the important parameters which should be monitored are those related to blood chemistry, particularly the oxygen, carbon dioxide and pH state of the patient's blood. A variety of techniques to monitor these parameters have been proposed and one particularly promising technique is the use of intravascular sensors which detect the analyte to be measured and transmit a signal proportional to the concentration of the analyte to a remote location. While there are a number of concepts that could be used to perform these functions, such a system can advantageously utilize the phenomena in which the output of certain fluorescent dyes is quenched in proportion to the concentration of an analyte present in a solution to which the dye is exposed. Such concepts have been extensively studied and developed in the context of an optical fiber inserted into the blood vessel and containing at its distal end a quantity of fluorescent dye which is irradiated by light of a first wavelength from a source at the proximal end of the fiber. The emissions from the dye are commonly at a frequency different from that used for excitation and the output of the dye is conducted through the optical fiber from the distal end to a detection means near the proximal end of the fiber. Such a system has been described in the literature and a number of United States patents. One particularly advantageous structure utilizing this phenomenon is that described in Buckles, U.S. Pat. No. 4,321,057, in which the sensor element is distributed along a portion of the optical fiber and the quenching of the signal occurs cumulatively along the length of the sensor element thus deployed. The calibration of such sensors prior to and during their use is a problem not easily addressed, and is particularly difficult if reagents are present which diminish the effectiveness of the sensor material or affect its linearity and sensitivity to the analyte to be measured. A variety of techniques have been proposed to assist in the calibration of the sensor during it's use, but one particularly persistent problem has been the degradation of the sensitivity of such sensors in the presence of halothane such as would be encountered in a patient to whom anesthetic has been administered. Since halothane, a common inhalation narcotic frequently used as an anesthetic during surgery, also quenches the fluorescent effect of oxygen sensitive indicator dyes, this has represented a serious issue for designers of intravascular oxygen sensor systems. Accordingly, it would be advantageous if a means or method could be found to substantially reduce or eliminate the effect of halothane upon the fluorescent indicators used for intravascular sensing of blood oxygen levels.